1. Field of the Invention
The present invention relates to an image forming apparatus and a method for simultaneously forming an image on more than one surface. More particularly, the present invention relates to an image forming apparatus and method for forming an image in which a digital color copying machine or a printer apparatus forms an image on more than one surface of an intermediate transfer medium at the same time.
2. Discussion of the Background
As shown in FIG. 6, a digital color copying machine including an automatic manuscript document feeding apparatus (hereinafter, called "ADF") for transferring the images on two surfaces to an intermediate transfer belt at the same time includes an ADF 1, a manuscript document reading-out apparatus 2, an image writing-in unit 3, an image forming unit 4, a primary transfer unit 5, a secondary transfer unit 6, and a fixing unit 7.
The ADF 1 sends the manuscript document to be read out to the manuscript document reading-out apparatus 2 and withdraws the manuscript document read out by the manuscript document reading out apparatus 2. The manuscript document reading out apparatus 2 includes a light source (not shown), a carriage 21 having plural mirrors (unnumbered) , a lens 22, a CCD 23, and a buffer 24. The apparatus 2 scans and reads out the manuscript document sent by the ADF 1 and sends the read-out image information to the image writing-in unit 3. The image writing-in unit 3 includes a laser light source (unnumbered), a polygon mirror (unnumbered), etc. (unnumbered), and sends a laser beam 31 including the image information to the image forming unit 4. The image forming unit 4 includes a charger 42 disposed around a photosensitive body 41, a radiating section for radiating the laser beam 31 from the image writing-in unit 3, a color developing section 43 of the rotating type or fixing type for developing Black (K), Cyan (C), Magenta (M), and Yellow (Y), and a drum cleaning section 44. The image forming unit 4 forms an electrostatic latent image with the laser beam 31 on the photosensitive body 41, the surface of which is charged by the charger 42; thereafter, the color developing section 43 visualizes the electrostatic latent image formed by the image forming unit 4. In such manner, a toner image is formed.
The primary transfer unit 5 includes an intermediate transfer belt 51, a primary transfer section 52, a tension roller 53, two pieces of the secondary transfer rollers 54 and 55, a cleaning section 56, and a standard position generating section 57. The intermediate transfer belt 51 can transfer the toner images onto two surfaces at the same time. The intermediate transfer belt 51 performs the primary transferring operation for the toner image on the photosensitive body 41 during an operational period. During the other periods, i.e., when not in the operational period, the intermediate transfer belt 51 is detached from the surface of the photosensitive body 41 by the action of an attaching/detaching mechanism (not shown). The belt 51 is pressed against the surface of the photosensitive body 41 only when the toner image is primarily transferred onto the intermediate transfer belt 51. A secondary transfer unit 6 secondarily transfers the toner image transferred onto the intermediate transfer belt 51 to a transfer material (not shown).
The intermediate transfer belt 51 is constructed so as to become larger (wider) than the maximum transfer paper size (for example, A3) in the copying machine of the present invention. If the transfer paper to be employed is equal to or smaller than the A4 size, the intermediate transfer belts 51 may transfer the toner image for two surfaces (e.g., two sheets of the transfer paper) at the same time.
The operation of forming the image (which is set in the manuscript document reading out apparatus 2 on the intermediate transfer belt 51) in the digital color copying machine constructed as mentioned above is explained with reference to FIGS. 7(a) and 7(b). In FIG. 7(a), the length L of one circumference of the intermediate transfer belt 51 is represented by the distance between the dashed lines.
As shown in FIG. 8, when the read-out image is read out on the full-color manuscript document 8 is formed, the manuscript document 8a set in the first reading out position 25 is scanned by the manuscript document reading-out apparatus 2. As shown in FIG. 7(a), the toner image 81K of Black (K) is transferred onto the intermediate transfer belt 51. Next, the manuscript document 8a is set on the primary reading-out section 25 is scanned again by the manuscript document reading out apparatus 2. Then the toner image 81C of Cyan (C) is transferred onto the intermediate transfer belt 51 which circulates once on the basis of a mark signal from the standard position generating section 57. This process is repeated for the toner image 81M of Magenta (M) and for the toner image 81Y of Yellow (Y) in this order. In such a manner, the full-color image is formed on the intermediate transfer belt 51. Then, the secondary transfer unit 6 transfers the full-color image onto the transfer material.
As shown in FIG. 9, a first full-color manuscript document 8a is set on the second reading-out position 26 at the downstream side of the manuscript reading-out apparatus 2. When a second full color manuscript document 8b is set on the first reading out position 25, the second manuscript document 8b and the first manuscript document 8a are respectively scanned a predetermined number of times by the manuscript document reading-out apparatus 2.
As shown in FIG. 7(b), the toner images 81K and 82K of Black (K) on the first manuscript document 8a and the second manuscript document 8b are transferred onto the intermediate transfer belt 51 at the time of the first scanning. In a similar way, the toner image 81C and 82C of Cyan (C), the toner images 81M and 82M of Magenta (M), and the toner images 81Y and 82Y of Yellow (Y) are respectively transferred onto the intermediate transfer belt 51, at the time of the second scanning, the third scanning, and the fourth scanning, respectively. In this manner, the full-color image of the first and second manuscript documents 8a and 8b is formed on the intermediate transfer belt 51. Then the secondary transfer unit 6 transfers the full-color image onto the transfer material.
In this manner, the full-color image of the first and second manuscript documents 8a and 8b is formed on the intermediate transfer belt 51.
However, when a black-and-white manuscript document and a full-color manuscript document are set on the manuscript document reading-out apparatus 2, it is necessary to execute at the same time the black-and-white image forming sequence and the full-color image forming sequence. This causes the control operations for the timing of scanning the manuscript document and for primarily transferring to become extremely complicated. Consequently, in conventional image forming apparatuses, the black-and-white image forming sequence and the full-color image forming sequence has to be independently executed, as shown in FIG. 10. Thus, the toner image 81K of Black (K) on the first manuscript document 8a is transferred onto the intermediate transfer belt 51, the toner image 81K is transferred onto the transfer material, thereafter, the toner image 82K of Black (K), the toner image 82C of Cyan (C), the toner image 82M of Magenta (M), and the toner image 82Y of Yellow (Y) are transferred onto the intermediate transfer belt 51 in this order, and as a result, the full-color image on the second manuscript document 8b is formed on the intermediate transfer belt 51.
Furthermore, in the case of forming the images on two sheets of the manuscript document which is sent from the ADF 1 to the manuscript document reading-out apparatus 2 and set thereon, the controls for the exchanging timing for the manuscript document by use of the ADF 1 and the timing of scanning the manuscript document may become very complicated. For this reason, in conventional image forming apparatuses, the image forming sequence has been independently performed for each of the respective manuscript documents.